Field of the Invention
The invention relates to an apparatus for conversion of electrical energy to heat in the field of drive and/or high-voltage technology by means of a braking resistance and at least one controllable braking power semiconductor for controlling the conversion.
By way of example, an apparatus such as this is already known from DE 10 2005 040 549 A1. The apparatus described there relates to a so-called multilevel converter, in which power semiconductor valves are connected between an AC voltage connection and a DC voltage connection. This results in a bridge circuit, which forms a positive and a negative DC voltage connection during normal operation. A series circuit of bipolar submodules extends between the positive and the negative DC voltage connections, and these bipolar submodules each have an energy store and a power semiconductor circuit. The power semiconductor circuit and the energy store are connected to the bipolar output of the submodule such that either the voltage dropped across the energy store or a zero voltage can be produced at the bipolar output of each submodule. A braking resistance is arranged in series with the series circuit of the submodules. The series circuit of the submodules and of the braking resistance is frequently also referred to as a braking controller. The positive and the negative DC voltage connections are connected via a DC voltage link circuit to a further converter which, for example, is operated as an inverter and is connected to an AC voltage power supply system or a polyphase motor. In the event of a fault, it is possible that the inverter may not be able to feed the real power produced on the DC voltage side into the connected AC voltage power supply system or polyphase motor. A situation such as this occurs, for example, during braking of the polyphase motor. The braking resistance is then used to convert the excess real power which occurs in a situation such as this to heat. This allows the converter, which is being operated as a rectifier, to still continue to operate as such, without this leading to total failure of the entire installation.
By way of example, FIGS. 1 to 4 show further apparatuses according to the prior art.
FIG. 1 shows three phase braking branches 1, which are connected on the AC voltage side to a converter. In this case, each phase braking branch 1 is conductively connected to an AC voltage connection of the converter. Furthermore, each phase braking branch 1 has a mechanical switch 2 and a braking resistance 3. At their end remote from the AC voltage connection, the phase braking branches 1 are connected to one another to form a common star point 4. By way of example, a mechanical circuit breaker, which is known per se, is used as the switch 2. When the circuit breaker 2 is switched on, current flows from the respective phase of the AC voltage connection via the braking resistance 3 to the common star point 4, with the electrical energy being converted to heat by the braking resistance.
FIG. 2 shows an exemplary embodiment as shown in FIG. 1, but with controllable power semiconductor valves 5 being chosen as the switches 2, and being connected back-to-back in parallel with one another. The power semiconductor valves 5 are, for example, thyristor valves, which cannot be turned off and are therefore externally commutated.
FIG. 3 shows a further exemplary embodiment according to the prior art. This figure also shows a converter 6 which has power semiconductor valves 7 which form a so-called six-pulse bridge circuit. Each power semiconductor valve 7 extends between an AC voltage connection 8 and a DC voltage connection 9 or 10. The DC voltage connections 9 and 10 are at different polarities during operation of the apparatus, and are therefore identified by a positive sign and a negative sign. A braking resistance 3 extends between the DC voltage connections 9 and 10, and is illustrated schematically here in the form of an inductance 11 and a non-reactive resistance 12. The braking resistance 3 is arranged on the DC voltage side of the converter 6, although the figure does not show a switch, which is arranged in series with the braking resistance, for connection of the braking resistance.
FIG. 4 shows an exemplary embodiment in which a series circuit 13 of power semiconductors which can be turned off and with freewheeling diodes connected back-to-back in parallel with them extends between the positive and the negative DC voltage connections 9 and 10 of a DC voltage link circuit. A braking resistance 3 is connected in series with the series circuit 13.
The apparatus of this generic type has the disadvantage that the time period between the activation of the braking controller and the effective conversion of real power to heat is too long to make it possible to reliably exclude faults on the converter.